The file consists of 3 layers, some created in illustrator and then painted in photoshop. The file opens and displays absolutely perfectly on my mac, however, when I send it to the clien they have reported back that when they open the file themselves, all they see is a blank image. Ive tried resaving and resending the file, but the problem still persists.

I've managed to sort out the visibility issue, but now they are reporting another!

The image I'm sending has been exported at 300dpi in A3 size. When I check the dimensions they are exactly what I see and the quality is perfect.

When they are opening it their end, they say that image is 72dpi and all layers have been flattened.

As the file is to be printed on clothing it obviously needs to be at a high dpi. I can't for the life of me figure out how the dpi they are seeing and the dpi I'm seeing are different?

The image was originally hand drawn, scanned into manga studio, inked, and then exported as a photoshop file. I've since live traced the image and turned it into a vector, placed it into photoshop and then added a gradient. There should surely be no issues right?

Are you by any chance using Apple's Mail program to send the files? If so, try making an archive of the files (zipping them) and send them a ".zip" file instead. Alternatively, use a free service such as http://www.yousendit.com to transfer the files.

Apple's Mail does all kind of things to image files, including downsizing them.